TrueType hinting problems

[Luc[as]]

It seems impossible to hint the widths of glyphs at certain PPMs (14, 18, 20, 22, 23, 25, 26, 28, 30, 31, 34 etc)
When you set a delta on the RSB @14 PPM, nothing happens to the width, until you additionally set a delta on the RSB @15 or @17 PPM, then the delta @14 suddenly has the desired effect.
And when you remove the deltas at other sizes, the delta @14 stops working again.
A problem that might be related is that when widths are on pixel center, FontLab rounds them often wrong compared to the MS TrueType rasteriser.
Both problems described happen on WinXP-Win7, with any rasteriser setting.
(so when FontLab is fixed, we have to check all TT hinted fonts :-)

[Luc[as]]

I found a workaround for the first problem: add ppms for which you need width adjustments to Device metrics table [hdmx]. :-)
Logically it doesn't make sense, in my opinion [hdmx] must be calculated at font generation time, after all hinting is done, and automatically include all PPMs for which non linear scaling has been used.
The FontLab default [hdmx] PPMs are from "recommendations for Windows fonts" from several decades ago. Today I have a lot of situations where this list does not make sense, e.g. when hinting for web or for bitmap fonts...

http://www.microsoft.com/typography/otspec/hdmx.htm
http://www.microsoft.com/typography/otspec/recom.htm#hdmx

Luc(as)

[Luc[as]]

Some anomalies occuring with the 2nd problem, wrong rounding of widths on pixel centers.

At 12 ppm, an advance width in FL rounds to the left (but should be right).

Anomaly 1: Identical width of composite glyph (e.g. agrave) rounds correctly.
Anomaly 2: Adding a delta (+8) to the RSB moves the width 2 pixels instead of 1.
Anomaly 3: Adding a delta to the RSB at any other size e.g. 10 ppm makes the glyph at 12 ppm suddenly round correctly. Scary :-)

Hope this helps to find bug.

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